The present invention relates to protective distance relays for use in a-c power systems, and more particularly to a distance relay employing an improved polarizing signal.
As a result of advancements in solid state technology, static relays have been developed which employ a phase comparator method of measurement. Exemplary circuits for phase comparator static relays are disclosed in U.S. Pat. No. 4,034,269, entitled "Protective Relay Circuits", issued July 5, 1977 to S. B. Wilkinson. This patent is assigned to the assignee of the present invention and is hereby incorporated by reference into the present application.
In the phase comparator method of measurement, the relay operates by measuring the angle between two or more phasors derived within the relay. Briefly, this is accomplished by the use of an a.c. coincidence circuit in combination with a timer circut. For example, in the case of the phase distance relay of FIGS. 6 and 7 of my previously referenced U.S. Pat. No. 4,034,269, a coincidence detector 286 develops an output signal when phase coincidence exists between a polarizing signal (V.sub.AB) and an operating signal [(I.sub.A -I.sub.B)Z.sub.R -V.sub.AB ]. A timer 288 then measures the duration of the phase coincidence and operates when phase coincidence occurs for a predetermined minimum time equal to the relay operate time. It is to be appreciated that, for faults within the intended reach of the relay, the phase coincidence is greater than 1/4 cycle. Thus, for 60 cycle protective relaying applications, where 1/4 cycle equals 4.167 ms, the timer is ordinarily set at 4.167 ms.
Although the above-described phase distance relay is widely employed, under certain unusual conditions, the relay may function improperly. In order to reduce the chances for such improper relay operation, phase distance relays have been developed which have a "variable mho characteristic" wherein the operating characteristic of the relay is a function of the power system conditions. That is, the characteristic has a diameter which is a function of the source impedance behind the relay and the relay reach setting. Such a "variable mho characteristic" is more fully explained on page 2 of my copending application of Ser. No. 879,653, filed Feb. 21, 1978, now U.S. Pat. No. 4,161,011 of which the present application is a continuation-in-part.
The "variable mho characteristic" is generally obtained by employing a polarizing voltage other than the faulted phase voltage, i.e., the faulted phase voltage for the phase AB relay would be V.sub.AB. Further discussion of polarizing voltages can be found in the article entitled, "Polarized mho Distance Relay", by Wedepohl in PROC. IEE, Vol. 112, No. 3, March, 1965. Polarizing voltages suitable for obtaining such a "variable mho characteristic" for the phase AB relay include: V.sub.AB1, representing the positive sequence component of the faulted phase voltage A.sub.AB ; and V.sub.AB +KV.sub.CN -90.degree., where K is a constant of relatively small magnitude so as to have a minimal effect on the response of a phase AB relay for faults involving the C phase.
Although the above-described polarizing voltages provide a "variable mho characteristic", there are still system conditions under which the phase relay may either function improperly, or, under which it may be difficult to define the limits of proper relay operation. For example, employing only the positive sequence component of the faulted phase voltage, V.sub.AB1, as the polarizing signal is unsatisfactory for those applications in which substantial load flow over the system is expected, especially with load flow in the tripping direction. This unsatisfactory result arises because the positive sequence voltage is largely a function of the prefault voltage at the relay while the fault current is largely a function of the prefault voltage at the source behind the relay. As the angle between the prefault voltage at the source and at the relay location increases, the likelihood of relay operation will decrease until this angle is sufficiently large to preclude relay operation. Similarly, employing V.sub.AB +KV.sub.CN -90.degree. as the polarizing signal presents a problem in establishing relay performance on faults involving the C phase, particularly on protected lines having heavy load flow and series compensation.
Accordingly, it is a general object of my invention to provide a distance relay having a polarizing signal which results in improved relay operation.
It is another object of my invention to provide a phase distance relay having a polarizing signal which results in improved relay operation.
It is another object of my invention to provide a ground distance relay having a polarizing signal which results in improved relay operation.